1. Field of the Invention
This invention relates to an electron source and an image-forming apparatus such as a display as an instance of application thereof, and more particularly, it relates to an electron source provided with a plurality of surface-conduction electron-emitting devices, and an image-forming apparatus such as an electronic display and a method of driving the same.
2. Related Background Art
Thermal cathodes and cold cathode electron sources are two known types of electron emitting devices, of which the latter include field-emission type (hereinafter referred to as FE type), metal/insulation layer/metal type (hereinafter referred to as MIM type) and surface-conduction electron emitting devices.
Examples of FE type devices are proposed in W. P. Dyke & W. W. Dolan, "Field emission", Advances in Electron Physics, Vol. 8, p. 89 (1956); and, A. Spindt, "Physical Properties of thin-film field emission cathodes with molybdenum cones" J. Appl. Phys., Vol. 32, p. 646 (1961).
An MIM type device is disclosed in C. A. Mead, "The tunnel-emission amplifier", J. Appl. Phys., Vol. 32, p. 646 (1961).
A surface-conduction type electron-emitting device is proposed in M. I. Elinson, Radio Eng. Electron Phys., p. 10 (1965).
A surface-conduction electron-emitting device utilizes the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elison proposes the use of an SnO.sub.2 thin film for a device of this type, the use of an Au thin film is proposed in G. Dittmer, "Thin Solid Films", 9, 317 (1971) whereas the use of an In.sub.2 O.sub.3 /SnO.sub.2 thin film and that of a carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf", 519 (1975), and H. Araki et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 43 of the accompanying drawings schematically illustrates a surface-conduction electron-emitting device proposed by M. Hartwell. In FIG. 43, reference numerals 431 and 432 respectively denote an insulator substrate and an H-shaped metal oxide film for electron-emission formed thereon by sputtering. Reference numeral 433 denotes an electron-emitting region that becomes operational when electrified in a process generally referred to as "forming", which will be described hereinafter. The entire thin film including the electron-emitting region is designated by numeral 434 in FIG. 43. For a device as illustrated in FIG. 43, L1 is between 0.5 and 1 mm and W is equal to 0.1 mm.
An electron-emitting region 433 is produced in a surface-conduction electron-emitting device normally by electrifying a thin film 432 for electron-emission on the device, a process generally referred to as "forming". More specifically, a DC voltage or a slowly rising voltage that rises, for instance, at a rate of 1 V/min. is applied to the opposite ends of the thin film 432 for electron-emission to locally destroy, deform, or structurally modify the thin film 432 for electron-emission to produce fissures in a part of the thin film, which constitute an electrically highly resistive electron-emitting region 433. Once the surface-conduction electron-emitting device is processed for forming, electrons will be emitted from those fissures and their neighboring areas when a voltage is applied to the thin film 434 including the electron-emitting region 433 to cause an electric current to flow through the device.
Known surface-conduction electron-emitting devices are, however, accompanied by problems when they are put to practical use. The applicant of the present patent application who has been engaged in the technological field under consideration has already proposed a number of improvements to the existing technologies in order to solve some of the problems, which will be described in greater detail hereinafter.
Surface-conduction electron-emitting devices are, on the other hand, advantageous in that they can be used in arrays in great numbers over a large area because they are structurally simple and hence can be manufactured at low cost in a simple way. In fact, many studies have been made to exploit this advantage and applications that have been proposed as a result of such studies include charged beam sources and electronic displays.
A large number of surface-conduction electron-emitting devices can be arranged in an array to form a matrix of devices that operates as an electron source, where the devices of each row are wired and regularly arranged to produce columns. (See, for example, Japanese Patent Application Laid-open No. 64-31332 of the applicant of the present patent application.)
As for image-forming apparatuses such as displays, although very flat displays comprising a liquid crystal panel in place of a CRT have gained polularity in recent years, such displays are not without problems. One of such problems is that a light source needs to be additionally incorporated into the display in order to illuminate the liquid crystal panel because liquid crystal does not emit light by itself. An emissive electronic display that is free from this problem can be realized by using a light source formed by arranging a large number of surface-conduction electron-emitting devices in combination with fluorescent bodies that are induced to selectively shed visible light by electrons emitted from the electron source. With such an arrangement, an emissive display apparatus having a large display screen and enhanced display capabilities can be manufactured relatively easily at low cost. (See, for example, U.S. Pat. No. 5,066,883 of the applicant of the present patent application.)
Incidentally, the emissive display apparatus of the above identified category comprising an electron source formed by a large number of surface-conduction electron-emitting devices and fluorescent bodies can be operated by drive signals that are applied to the wires connecting the respective surface-conduction electron-emitting devices arranged in rows (row wires) and to the control electrodes arranged in the space separating the electron source and the fluorescent bodies along a direction perpendicular to the row wires (grids or column electrodes). (See, for example, Japanese Patent Application Laid-open No. 1-283749 of the applicant of the present patent application).
There are, however, a number of difficulties that have to be overcome before such a display apparatus becomes commercially feasible. Some of the difficulties include the problem of accurately aligning individual surface-conduction electron-emitting devices and corresponding individual grids and the problem of securing a uniform distance between each grid and the corresponding surface-conduction electron emitting device, both of which are manufacture-related problems. In an attempt to solve these manufacture-related problems, there has been proposed an improved display apparatus of the category under consideration, in which the grids are formed into a layer and laid on the layer of the surface-conduction electron-emitting devices to produce a multilayer structure. (See, for example, Japanese Patent Application Laid-open No. 3-20941 of the applicant of the present patent Application.)
FIGS. 44 and 45 illustrate a known typical electronic display comprising conventional surface-conduction electron-emitting devices as disclosed Japanese Patent Publication No. 45-31615. Referring to FIGS. 44 and 45, it comprises transversal current type electron-emitting bodies 442 connected in series, strip-shaped transparent electrodes 444 arranged perpendicularly to the electron-emitting bodies 442 to form a lattice therewith and a glass panel 443 provided with a number of small holes 443' and disposed between the electron-emitting bodies and the electrodes in such a manner that the holes are located on the respective crossings of the electron-emitting bodies and the electrodes. Each of the holes 443' contains gas hermetically sealed therein so that the display emit light by gas-electric discharge only at the crossings of those transversal current type electron-emitting bodies 442 that are currently discharging electrons and those transparent electrodes 444 to which an accelerating voltage E2 is currently being applied. While Japanese Patent Publication No. 43-31615 does not describe the transversal current type electron-emitting body in detail, it may safely be presumed that it is a surface-conduction electron-emitting device because the materials (metal thin film, mesa film) and the structural features of the neck 442' described there exactly match their counterparts of a surface-conduction electron-emitting device. For the purpose of the present invention, the term "surface-conduction electron-emitting device" is used in the sense as defined in "The Thin Film Handbook".
Now, some of the problems that have arisen with electronic displays comprising known surface-conduction electron-emitting devices will be discussed below.
Three major problems have been pointed out for a display apparatus disclosed in the above cited Japanese Patent Publication No. 45-31615.
(1) While the display apparatus is designed to operate for electric discharge as electrons emitted from the transversal current type electron-emitting bodies are accelerated and caused to collide with gas molecules, the pixels of the apparatus can glow by electric discharge with different levels of luminance and the luminance of a same pixel can fluctuate when the transversal current type electron-emitting bodies are energized to a same intensity. One of the possible reasons for this may be that the intensity of electric discharge of such an apparatus is heavily dependent on the state of the gas in the apparatus and not satisfactorily controllable, while another may be that the output level of a transversal current type electron-emitting body cannot necessarily be stabilized if the gas pressure is somewhere around 15 mmHg as described in the Examples section of the cited patent document.
Thus, the above described display apparatus is not able to provide any multiple-tone display and therefore can offer only a limited scope of use.
(2) While the display apparatus can change the color for display by using a different type of gas, the use of various gases does not necessarily extend the scope of color display because the wavelength of visible light generated by electric discharge does not cover a wide range. Additionally, the optimum gas pressure used for the emission of light by electric discharge varies as a function of the type of gas involved.
Thus, in order to achieve a color display by using a single panel, different gases must be sealed in the holes with varied gas pressures depending on the locations of the holes, making the manufacture of such an apparatus extremely difficult. If, for example, three laminated panels are used for a display apparatus to avoid this problem, it will become unrealistically heavy and the manufacturing cost will be prohibitive to produce such a heavy apparatus.
(3) Since the display apparatus comprises a large number of components including the substrates of the transversal current type electron-emitting bodies, the strip-shaped transparent electrodes and the holes where gas is hermetically sealed, it is structurally very complicated and hence only a very small error margin is allowed for aligning the components. Additionally, since the threshold voltage used for the emission of light by electric discharge is as high as 35 [V] as described in the cited document, each electric element used in the panel drive circuit is required to show a high withstand voltage.
Thus, manufacturing such a display apparatus is complicated and expensive.
It is mainly due to the above reasons that an electronic display of the above described type has not been able to find any practical applications in the field of television receiving sets and other similar electronic apparatuses.
On the other hand, the image-forming apparatuses proposed by the applicant of the present patent application and comprising an electron source formed by arranging a number of surface-conduction electron-emitting devices and a same number of fluorescent bodies juxtaposed therewith are not without problems.
Firstly, in order to realize such an electron source, it is indispensable to arrange grids along a direction (column-directed wiring) perpendicular to the wires connecting the electron-emitting devices arranged in parallel (row-directed wiring) if the devices are selectively made to emit electrons. In this regard, no simple and easy process has been developed for manufacturing an electron source with which devices are selected for the emission of electrons and the level of electron emission is controllable.
Secondly, in order for the fluorescent bodies of such an image-forming apparatus arranged in juxtaposition with the electron source to emit light at selected locations with a controlled level of luminance, a certain number of grids need indispensably be provided as in the case of the electron source. Again, no simple and easy process has been developed for manufacturing an image-forming apparatus comprising such fluorescent bodies, with which electron-emitting devices can be selected with difficulty to cause them emit light at a controlled level according to incoming signals so that the fluorescent bodies may be made to glow at selected locations with a controlled level of luminance.